Prototyping for aerospace and defence: choosing the right process for your parts

Choosing a prototyping process is rarely straightforward. In aerospace and defence work, the stakes are higher still – parts need to perform reliably in testing, materials need to meet programme requirements, and there often isn’t time to get it wrong.

This guide is written for engineers and project leads who already know the basics and need practical guidance on which process best fits a specific application. It covers CNC machining, SLA, SLS and vacuum casting – when each is the right call, where the trade-offs are, and how to think about the decision when more than one option looks viable.

The key questions to answer first

Before comparing processes, it’s worth pinning down a few things that will narrow the field quickly:

What will the part actually do in testing?
A structural bracket that will be load-tested needs different properties from an avionics housing that only needs to be checked for fit and form. Functional testing pushes you toward CNC or high-performance resins. Fit and form evaluation opens up more options.

What material properties matter? 
Temperature resistance, tensile strength, weight, and chemical resistance – different processes give you access to different materials. If you need PEEK, you need CNC machining. If you need a flame-retardant grade for an enclosure, certain DLP resins can deliver that.

What geometry are you working with? 
Complex internal channels, organic forms and lightweight lattice structures favour additive processes. Tight-tolerance bores, threaded features and flat reference surfaces are better served by CNC.

How many parts do you need, and how quickly? 
A single prototype for a design review has different economics than a batch of 20 test components. And a three-week programme window changes the conversation compared to a three-day deadline.

CNC machining: when accuracy and material properties are non-negotiable

CNC machining – milling and turning – is often the default choice in aerospace and defence prototyping, and for good reason. It produces parts in real engineering materials, to tight tolerances, with a surface finish and mechanical behaviour close to what a production part would deliver.

When CNC is the right choice

Structural test components that will be load-tested or fatigue-tested
Parts with tight tolerances – ±0.05 mm is achievable, tighter on specific features
Applications where material certification matters (aluminium 7075, stainless steel 316, PEEK)
Threaded features, precision bores, and flat reference surfaces
Jigs, fixtures and ground support equipment

Material highlights for this sector
Aluminium 7075 is the go-to for high-strength, lightweight structural parts. Aluminium 6082 is a more economical choice for general-purpose brackets and fixtures where the extreme strength of 7075 isn’t required. PEEK handles high temperatures and aggressive environments where most polymers would fail. Stainless steel 316 is worth the step up from 304 when corrosion resistance is a genuine requirement rather than a precaution.

The trade-off
CNC machining is process-constrained by geometry. Deep undercuts, complex internal channels and organic surfaces that would be straightforward in additive manufacturing can add significantly to cost and lead time – or simply not be possible from a billet. If your part has geometry that would be difficult to fixture and machine, it’s worth considering whether additive manufacturing can meet your performance requirements instead.

At Prototype Projects, CNC parts are available from three working days, with 5-axis milling capability for complex geometries.

SLA 3D printing: large, accurate parts for evaluation and mock-up

Stereolithography (SLA) produces resin parts with a smooth surface finish and good dimensional accuracy – qualities that make it well-suited to aerospace prototyping where fit, form and visual evaluation matter, and where part size can be a challenge.

When SLA is the right choice

Full-scale system mock-ups and enclosures for evaluation
Avionics housing prototypes where fit checks and cable routing need to be verified
Parts where surface finish is important for presentation or evaluation
Large components – our SLA 750 machine accommodates parts up to 750 x 650 x 550 mm
Applications requiring optical clarity (canopies, sight components, light pipes)

Material highlights for this sector
Matrix HT300 (an Accura resin) is a high-temperature resin that retains its properties above 200°C under load – relevant for parts tested in or near heat sources. Standard Accura resins give good stiffness and surface quality for the majority of evaluation applications. Clear SLA resins are available where transparency is required.

The trade-off
SLA resins are not structural materials in the way that machined metals or PEEK are. They’re well-suited to evaluation, fit checks and functional testing at moderate loads – but if a part is going to be subjected to significant mechanical stress in testing, CNC machining or SLS is likely a better fit. SLA parts can also be sensitive to prolonged UV exposure, which is worth bearing in mind for parts that will be stored or used outdoors.

SLS 3D printing: functional nylon parts without tooling

Selective laser sintering (SLS) produces parts in nylon (PA12) that are genuinely functional – not just representative. No support structures are required during the build, which means complex geometries, internal channels and assembled-in-place features are all achievable without additional cost or lead time.

When SLS is the right choice

Lightweight structural components where metal isn’t required
Complex ducting, cable management and internal structures
UAV, drone, AUV, ROV and UGV components where complex geometry and weight distribution are key considerations
Parts with internal features that would be inaccessible or expensive in CNC
Batches of functional test parts, needed quickly

Material highlights for this sector
Standard PA12 is strong, slightly flexible, and resistant to a range of chemicals. At Prototype Projects, we offer Matrix PA12 CR, a carbon-reduced grade that offers equivalent mechanical performance with a lower environmental footprint.

The trade-off
SLS parts have a slightly grainy surface texture as-sintered, which can be improved with dyeing, surfacing or lacquering. Dimensional accuracy is good, but not at the level of CNC machining; if your part has features requiring ±0.05 mm, CNC is the better route. SLS is also constrained to nylon-based materials, which limits options if high-temperature performance or specific mechanical properties are required.

Overnight dispatch is available for SLS (and SLA) parts, making it a strong choice when programme timelines are tight.

Vacuum casting: small batches of representative parts

Vacuum casting uses silicone tooling made from a master pattern to produce small batches of polyurethane parts with properties that closely approximate injection-moulded polymers. It sits in a useful middle ground – more representative than most 3D printed parts, faster and cheaper than hard tooling.

When vacuum casting is the right choice

Batches of 5 to 50 evaluation or test parts
Parts that need to closely represent a production-intent material (flexible, rigid, overmoulded)
Enclosure and housing evaluation, where fit to adjacent components, needs to be checked across multiple samples
Programmes moving toward production, where a bridge solution is needed before tooling is ready

The trade-off
Vacuum casting requires a master pattern to be produced first, which adds a step and some lead time. It’s less suited to single-part requirements or very early-stage prototyping where geometry is still changing. The silicone tooling has a limited life – typically 20 to 25 shots – so for larger volumes, injection moulding becomes more economical.

When the journey continues: from prototype to production

Some aerospace and defence programmes run extended low-volume production series before a product reaches full-scale manufacture. If your programme is heading in that direction, it’s worth thinking about the route early.

Injection moulding with UK-made tooling is the step that follows prototyping for plastic parts heading into production. Having the same supplier (such as Prototype Projects) manage the prototyping and the transition to moulded parts reduces the risk of losing knowledge between stages, and means your toolmaker already understands the part.

A note on inspection

Whatever process you choose, dimensional inspection and inspection reports are available alongside your parts. For aerospace and defence applications, where traceability and verification are part of the quality process, this means you’re not sending parts for test without the documentation to back them up.

Not sure which process is right?

That’s not unusual – many of the most interesting parts in this sector sit on the border between two or three options. Our team is used to working through these decisions with engineers before a quote is submitted.

Contact us to discuss your project, or request a quote and and tell us what you’re working on. We’ll recommend the right approach and give you a clear, fast turnaround on pricing.